The present invention generally relates to programmable devices and methods of testing programmable devices, and more particularly to a programmable device having programmable cells and a method of testing such a programmable device.
Recently, there are field programmable devices which are easily programmable at the time of designing each device. An example of the field programmable device is a programmable read only memory (PROM).
FIGS. 1A through 1D are cross sectional views for explaining production processes of a single programmable cell of the conventional PROM using a fuse.
In FIG. 1A, an N.sup.+ -type buried layer 18 is formed on a P-type semiconductor substrate 11, and an N-type epitaxial layer 12 is formed on the N.sup.+ -type buried layer 18. V-grooves 13 are formed in the N-type epitaxial layer 12 and reaches the N.sup.+ -type buried layer 18. In addition, an N.sup.+ -type collector region 23 is formed in the N-type epitaxial layer 12 and extends to the N.sup.+ -type buried layer 18.
In FIG. 1B, a P-type base region 14 is formed within each region of the N-type epitaxial layer 12 isolated by the V-grooves 13.
In FIG. 1C, a SiO.sub.2 oxide layer 16 is formed on the N-type epitaxial layer 12, and a window 16a is formed in the oxide layer 16. An N-type emitter region 15 is formed in the P-type base region through the window 16a.
In FIG. 1D, an emitter electrode 17 is formed on the N-type emitter region 15.
A write operation with respect to the programmable cell shown in FIG. 1D is carried out by passing a current between the emitter and collector of the programmable cell. That is, a reverse current flows to the P-type base region 14 from the N-type emitter region 15 so as to break down a PN junction thereof.
However, when a positioning error of the mask or the like occurs, the N-type emitter region 15 may become close to the V-groove 13 and a gap between the N-type epitaxial layer 12 and the N-type emitter region 15 may become extremely small as shown in FIG. 2. In this case, a current flows in a vicinity of a boundary between the programmable cell and the V-groove 13 when carrying out a write operation with respect to the programmable cell, thereby generating an eutectic 19 of semiconductor and aluminum of the emitter electrode 17. As a result, the N-type emitter region 15 and the N-type epitaxial layer 12 becomes short-circuited, that is, an overprogramming occurs. Therefore, the programmable cell is no longer a diode, and it is impossible to carry out the write operation accurately.
For this reason, when the programmable device is produced, it is necessary to test the programmable device for deficiencies existing therein. However, because it is impossible to directly test a real cell region in which the actual programming takes place, the test is made by use of a cell region other than the real cell region and peripheral circuits such as a write circuit of the programmable device.
FIG. 3A is a block diagram showing the general construction of the conventional programmable device, and FIG. 3B shows a portion of the conventional programmable device on an enlarged scale.
In FIG. 3A, the programmable device has a real cell region 20, a test cell row 21 made up of a row of cells, and a test cell column 22 made up of a column of cells. The real cell region 20 is made up of m rows by n columns of cells arranged in a matrix arrangement. Although not shown, word lines are provided along a direction in which the columns of cells extend and bit lines are provided along a direction in which the rows of cells extend.
The test cell row 21 and the test cell column 22 are used exclusively for testing the write operation of the programmable device, and although not shown, there are also provided test cell row and column used exclusively for testing the read operation of the programmable device. For convenience sake, a description of the test cell row and column used exclusively for testing the read operation of the programmable device will be omitted. The test cell row 21 is made up of cells having the same construction as the rows of cells in the real cell region 20, and the test cell column 22 is made up of cells having the same construction as the columns of cells in the real cell region 20, assuming that there is no positioning error of the mask during the production of the programmable device. Hence, by testing the cells in the test cell row 21 and the test cell column 22, it is possible to essentially test the cells in the real cell region 20.
In FIG. 3B, a cell column 26 is provided between deep V-grooves 25. The cell column 26 has two cells 28A and 28B between collector contact portions 27A and 27B and these two cells 28A and 28B are isolated by the V-grooves 13 which are shallow compared to the V-grooves 25. The cross sectional view shown in FIG. 1D is taken along a line ID--ID in FIG. 3B, and in FIG. 3B, those parts which are the same as those corresponding parts in FIG. 1D are designated by the same reference numerals.
In the cell column 26, the cell 28A located below the collector contact portion 27A and the cell 28B located above the collector contact portion 27B have symmetrical shapes about the V-groove 13 formed between the cells 28A and 28B. For this reason, when a positioning error of the mask occurs in a direction along the cell colomn 26, the shapes of the cells 28A and 28B do not necessarily become the same.
The conventional programmable device such as the PROM only has one test cell row 21 and one test cell column 22 as shown in FIG. 3A. Hence, when a positioning error of the mask occurs during the production of the programmable device and the test cell row 21 is made up of a plurality of the cell 28A located below the collector contact portion 27A shown in FIG. 3B, for example, the test cannot be carried out with respect to the cell 28B located above the collector contact portion 27B. As a result, there is a problem in that the test cannot be carried out with respect to all of the cells in the real cell region 20.